Dependency injection in ruby with example

Today we will talk about dependency injection.
We all know that tighly coupled code is bad and code smells.
So, dependency injection helps us avoid such problems.
Let me give you an example of the following code.

class Car
  attr_reader :mileage
 
  def initialize(mileage)
    mileage = mileage
  end
  def worn_out
    mileage * Tire.new(“Maxis”, 60, 34).radius
  end
end
class Tire
  attr_reader :brand, :radius, :width
  def initialize(brand, radius, width)
    brand = brand
    width = width
    radius = radius
  end
end

As you see, we create a new instance of tire inside worn_out method.
If we change the name of Tire class or parameters number, we have to change the name of it inside that method too.
This is a tightly coupled code, classes depend on each other. If we change the class name or methods of Tire we also need to change the Car class.

Let’s solve this problem using dependency injection:

class Car
  attr_reader :mileage, :tire
  def initialize(mileage, tire)
    mileage = mileage
    tire = tire
  end
  def worn_out
    mileage * tire.radius
  end
end
class Tire
  attr_reader :brand, :radius, :width
  def initialize(brand, radius, width)
    brand = brand
    width = width
    radius = radius
  end
end

Car expects a ‘Duck’ that knows ‘radius’

Car.new(20000, Tire.new(“Maxis”, 60, 34))

In this example code, we injected the tire object into the constructor.
So Car class doesn’t depend on Tire class much.
If we change the Car class name or its parameters, it will not affect other classes.

Also, there is the Inversion of Control principle from SOLID created by Uncle Bob. According to this principle, code receives the flow of control from outside instead of being responsible to create it itself.